Rare earth-iron-boron based magnets, such as the well known Nd--Fe--B magnets, are used in numerous applications, including computer hardware, automobiles, consumer electronics and household appliances. In particular, magnets using rare earth elements, such as Nd or Pr, are useful primarily because of their superior magnetic properties, as manifested by their large coercivity, remanence, magnetization and maximum energy product. The primary disadvantage of such magnets is that because of the cost of scarce rare earth metals, such as Nd or Pr, they are relatively expensive to make.
Alternative, inexpensive permanent magnets, such as the ferrite-based magnets, which have long been available, generally exhibit poor magnetic properties as compared with rare earth based magnets. Thus, they are unsuitable for many applications. For example, the remanence of a permanent magnet fabricated from sintered ferrite is typically about 4.0 kG while the remanence for a bonded Nd--Fe--B magnet is about 6.5 kG. It would be useful to fabricate a bonded permanent magnet with a remanence that is intermediate between these values, of about 5.0 to 6.0 kG, which is sufficient for many applications for which ferrite magnets are unsuitable to use. In such cases, it would be particularly useful if such magnets could be fabricated at a lower cost than what is currently required for bonded Nd--Fe--B magnets.
U.S. Pat. No. 4,765,848 ("the Mohri patent") discloses a permanent magnet composition [(Ce.sub.x La.sub.1-x).sub.y R.sub.1-y ].sub.z (Fe.sub.1-v B.sub.v).sub.1-z where R is one or more rare-earth elements, excluding Ce and La. In the disclosure of this patent, restrictions have been placed on the values of x, y, z and v: 0.4.ltoreq.x.ltoreq.0.9; 0.2&lt;y&lt;1.0; 0.05.ltoreq.z.ltoreq.0.3; and 0.01.ltoreq.v.ltoreq.0.3. With such limitations, the patent discloses that the coercivity of the resulting permanent magnet is greater than 4 kOe. This coercivity is viewed as an appropriate index for providing a useful magnet, and the patent teaches that the coercivity is insufficient when the values of the various constituents fall outside the ranges specified above.
One of the principal teachings of the Mohri patent is that there is a synergistic effect when both Ce and La are used in rare-earth iron-boron magnets. According to the teaching of the Mohri patent, both Ce and La, when used alone, decrease the magnetic properties, but when used togther, the synergistic effect acts to enhance the coercivity.